handlebar grip

is it a safe assumption that after 30 or so years of sitting on some bars that the grip should go? im sure its the original and i dont want to destroy something so awesome, well its not that awesome, but its my first bike.

well then how do you know when you need to replace it? i went out last night after a shower and had some post shower lotion on the hands (no no no not what you guys are thinking!!) and it was as if i had no grips at all. whens the tell tale signs the grip is done for?

I am assuming we are talking about an upright style bike with grips like this style:

As long as they stay in place on the bar and don't move around they should be fine right? I guess if they are all torn up or have excessive scuffs and holes they could be replaced, but I don't think they really have to be.